For the electronic testing of devices having C4 bump terminal metallurgy, the test contacts must be shaped in such a way as to provide ohmic contact with the C4s being tested while applying minimal force so as to avoid causing damage. Such contact must be made simultaneously with a large number of bumps, making control of the shape, distribution and dimensions of the probes a priority. Fabrication of such contacts has in the past been affected by problems, including lack of uniform mask conformity to the surface, chipping and cracking of polymer, difficulty of maintaining adhesion of photoresist to copper surfaces and limited resist shelf life between certain process steps. Furthermore, in a manufacturing environment, the number of process steps affects cost and must be minimized. To overcome the above mentioned and other problems, the present invention includes only one photolithographic masking step in the fabrication of sculpted, uniformly shaped contacts of predetermined distribution and dimensions, using a mask of unique design. The mask itself is comprised of two or more sizes of features, through which differing etch rates can be achieved in the areas of copper underlying areas in the photoresist that have been exposed using the mask and then developed. As a result, complete removal of metal is achieved in some areas, no removal in others and partial removal in other areas. The phenomenon producing this effect is called herein gray scale etching. This invention also includes use of a novel part handling fixture in the gray scale etch process which is advantageous for scaleup to a high volume manufacturing process.
U.S. Pat. No. 4,374,911 to Phillip A Hartley and assigned to the assignee of the present invention describes the fabrication of a photolithographic emulsion mask having at least three zones of light transmissibility. The variation in light transmissibility in the mask is referred to in the patent as "gray scale masking". The gray scale mask is made as a composite image from superimposing the images of at least two preliminary masks, a first one of which has a pattern of greater light transmissibility and a second diazo mask of lesser transmissibility which blocks in part selected portions of the first mask.
The present invention is not concerned with creating more than two areas of varying light transmissibility in a mask. In the present invention, all areas of the mask which transmit light transmit the same amount of light; all exposed areas of resist are totally exposed and developed, and all areas of the mask intended to transmit light radiation do so completely. The present invention uses the phenomenon of differential etch rate of a metal through varying dimensions and distribution of openings, the smaller openings etching more slowly than the larger, meaning that the larger openings can be completely etched while the smaller can be only partially etched and a sculpted effect is created.
In U.S. Pat. No. 4,985,116, Mettler et al describe a mask making process whereby a flexible plastic sheet, patterned in ink, is vacuum molded to conform to the surface of a three dimensional substrate, such as a printed wiring board or connectors, and is exposed to a YAG laser, which removes the plastic from ink-free areas, creating a removable, flexible mask for applying to the surface of the substrate for subsequent plating or etching in the exposed areas. The conformable mask, intended to replace rigid machined masks, provide improved image resolution in lines of about 20 and in spaces of about 30 mils. Alignment marks on the mask provide registration assistance; vacuum maintains physical contact of the mask to the substrate. The use of a mask and gray scale etching, or any process or fixture to control shape, height and distribution of microminiature contacts or other surface features as in the field of the present invention is not addressed.
In U.S. Pat. No. 5,020,217, Raphael A. Gonzales et al describe a method for providing contacts of preselected height and shape using discrete deformation of a metal sheet. Alternatively, the contacts are made by extrusion. The described process is mechanical, using tool and die, unsuitable for dimensions demanded in the field of the present invention, in which chemical processing is used to achieve microminiature contacts. Similarly, the problem addressed in the reference, i.e. avoiding the presence of foreign particles on the contacts of a "push-button" switch and like large devices, is not relevant to the present invention.
In U.S. Pat. No. 5,071,787, inventors Miki Mori et al describe several approaches to creating a sound electrical/mechanical bond between a wiring layer on an insulating substrate and a mounted semiconductor device bonded face down thereto. Contacts between the two levels is made though a set of conductive bumps on each, and through controlled heating up just below melting point of one set while the two sets are pressed together, plastic deformation removes any oxide that may interfere with the quality of the electrical contact at the joint. As described alternatively, one set of bumps on the semiconductor side may be sufficient, provided the composition of the bumps possesses a sufficiently low melting point. Since there is not yet hardened resin fixing the levels in place, any electrical repair that may be required after testing is more easily accomplished and the expensive alternative of having to discard the device rather than to repair it is avoided. The present invention is not addressed to semiconductor device mounting per se, but to sculpted microminiature probes for testing and the formation thereof by gray scale etching.
In U.S. Pat. No. 5,207,585 to Herbert P. Byrnes et al. and assigned to the same assignee as the present invention is described the thin interface pellicle, also referred to as the thin flexible interposer (TFI), and electrical contacts thereon for testing. The '585 patent is highly relevant to the present invention and is incorporated herein by reference. The present invention, however, is directed to the electrical contacts and the process of fabricating the electrical contacts rather than to the thin interface pellicle.
Of particular interest in the '585 patent is the description of the "Cobra Probe". The Cobra Probe typically requires cleaning after testing about every 4-5 wafers before reuse. Because of the constant demand for greater and greater density electronic devices, there is a need for fabrication of test devices that can handle densities higher than those tested by the Cobra Probe. The limitation of usefulness of the Cobra probe appears to be 4 on 8, meaning that the bumps to be tested are 4 mils in diameter and the distance between bumps, center to center, is 8 mils, the gap being 4 mils. This is expressed as a "pitch" of 4 on 8. A gap of less than 4 mils, involving a tighter pitch, has more stringent testing requirements than a Cobra type of probe appears able to supply. The needle point probe of Cobra, which is long and rather flexible, can be used up to about 200 MHz. The length of the Cobra Probe is prone to an impedance unsuitably high in the more aggressive environment of the future. The probes of the present invention are particularly suited for future stringent test requirements.
European Patent application (EPA) 90124611.6 to Yoshinari Takayama et al describes a conductive material independently piercing an insulating film in through-holes and having a bump-like projection layer larger than the through-hole to keep the conductive material in place therein. The EPA does not address the problem of making ohmic contact at a bump surface through an oxide layer, any advantages of a sculpted probe or the like. Etching described in the EPA is of the through-hole in an insulating film preparatory to the provision of a conductive path therethrough and of the conductive layer laminated on the insulating film. The bump-like projections are made e.g. by plating, not by etching. None of the above references addresses the problems, achieves the results or discloses the processes and apparatuses of the present invention.